1. Field of the Invention
The present invention relates to a vibration damping bushing adapted to provide vibration damping linkage to two components for which vibration transmission is to be prevented in an automobile, or alternatively adapted to provide vibration damping mounting of one of the two components on the other in a vibration damping fashion. The present invention also relates to a method of producing the same.
2. Description of the Related Art
Conventionally, suspensions for automobiles and the like have commonly employed vibration damping bushings composed of a main shaft member, a rubber elastic body of tubular shape affixed to the outside peripheral face of the main shaft member, and a medial sleeve of tubular shape embedded coaxially in the medial region of the rubber elastic body in the direction of its thickness. This vibration damping bushing is of press fit design, intended to be used by press-fitting the rubber elastic body directly into a mounting hole, or press-fitting the rubber elastic body into an outer tube member, then press-fitting the outer tube member into a mounting hole.
As disclosed in JP-A-9-72365 and JP-A-11-141612 for example, vibration damping bushings of this kind are manufactured typically through vulcanization molding of a rubber material together with the main shaft member and the medial sleeve which have been positioned in the mold, thereby integrally molding the rubber elastic body with these components. The mold assembly used for this process is composed, for example, of an upper mold and a lower mold positioned spaced some distance apart in the vertical direction, and intermediate molds split into several sections in the circumferential direction and positioned extendably and retractably in the centripetal direction between the upper and lower molds.
Where a vibration damping bushing is manufactured using this type of mold assembly, first, the main shaft member and the medial sleeve are disposed resting upright (i.e. oriented with the axial direction aligned with the vertical) and disposed concentrically, with a first end thereof (the lower end) positioned on a positioning member disposed on the upper face of the lower mold of the mold assembly. Next, the lower mold is transported to a prescribed location, the intermediate molds are moved in the centripetal direction and set with respect to the lower mold, then again transported to a prescribed location where the upper mold is set and the mold assembly is locked. The mold assembly is then moved to the location of the vulcanizer where the vulcanizer injects rubber material into the mold cavity and carries out vulcanization molding. Then, prescribed post-processing steps are carried out to complete the vibration damping bushing.
Where a vibration damping bushing is manufactured in the above manner, the main shaft member and the medial sleeve are positioned concentrically by a positioning member disposed on the upper face of the lower mold of the mold assembly. At this time, while the main shaft member can be held in stable condition by the positioning member, for structural reasons portion of the medial sleeve held by the positioning member (i.e. the portion projecting from the axial end face of the rubber elastic body) is insufficient. Moreover, since there is dimensional variation (tolerance) of the medial sleeve, it is necessary to leave a gap (gutter) in the diametrical direction between the medial sleeve and the positioning member, so that the sleeve tends to be held in an unstable condition.
Thus, when the lower mold is transported to the prescribed location for the next process, the medial sleeve can easily shift out of position, for example, assuming a tilted state, or running up onto the positioning member. If the medial sleeve should shift out of position in this way, when the intermediate molds and the upper mold are set, the mold assembly or medial sleeve may become broken or deformed. Also, if vulcanization molding is carried out with the medial sleeve shifted out of position, dimensional accuracy of the product will be poor so that characteristics vary appreciably, or defective products may occur.
In order to prevent the medial sleeve from shifting out of position in this way, it may be contemplated to extend the length of the medial sleeve projecting beyond the axial end face of the rubber elastic body. However, since it is necessary to avoid interference with other components situated in proximity to the vibration damping bushing, there is a limit as to how much the length of the medial sleeve can be extended.